Power Electronics Multiple Choice Questions: Part 3

MULTIPLE CHOICE QUESTIONS BASED ON POWER ELECTRONICS AND DRIVES

[1] When a line commutated converter operates in the inverter mode
(a) It draws both real and reactive power from the AC supply
(b) It delivers both real and reactive power to the AC supply
(c) It delivers real power to the AC supply
(d) It draws reactive power from the AC supply

Answer: C

[2] A chopper operating at a fixed frequency is feeding an RL load. As the duty ratio of the chopper is increased from 25% to 75%, the ripple in the load current is
(a) Remains constant
(b) Decreases, reaches a minimum at 50% duty ratio and then increases
(c) Increases, reaches a maximum at 50% duty ratio and then decreases
(d) Keeps on increasing as the duty ratio is increased

Answer: A

[3] To turn off a SCR, the reverse bias should be applied for a period ……. the turn-off time of the SCR
(a) Equal to
(b) Longer than
(c) Less than
(d) Irrespective of

Answer: B

[4] In class A and class B commutation the resonating circuit has to be
(a) Over damped
(b) Critically damped
(c) Under damped
(d) Negatively damped

[10] A 100VDC is applied to the inductive load through a SCR. The SCR’s specified latching current is 100mA. The minimum required width of gating pulse to turn on the SCR is
(a) 100μS
(b) 100S
(c) 1mS
(d) 50μS

Answer: A

[11] A cycloconverter is a
(a) Frequency changer from higher to lower frequency with one-state conversion
(b) Frequency changer from higher to lower frequency with two-stage conversion
(c) Frequency changer from lower to higher frequency with one-stage conversion
(d) Either a or c

[13] In synchronized UJT triggering of an SCR, voltage VC across capacitor reaches UJT threshold thrice in each half cycle so that there are three firing pulses during each half cycle. The firing angle of the SCR can be controlled
(a) Once in each half cycle
(b) Thrice in each half cycle
(c) Twice in each half cycle
(d) Four times in each half cycle
(e) None of the above

Answer: A

[14] In a GTO, anode current begins to fall when gate current
(a) Is negative peak at time t=0
(b) Is negative peak at t = storage period tS
(c) Just begins to become negative at t = 0
(d) Is negative peak at t = (tS + fall time)